1. Field of the Invention
The present invention relates to a solid-state image pickup device and method for manufacturing a solid-state image pickup device having imaging region which is covered with transfer electrodes.
2. Description of the Prior Art
In a frame transfer-type CCD solid-state image pickup device, a image pickup section for receiving light from an object acts to accumulate information charges generated in response to the illuminated light and simultaneously transfers the information charges having been accumulated for a predetermined time period to an accumulating section. Therefore, transfer electrode is provided on the light-receiving region to transfer the information charges.
FIG. 1 is a plan view showing outline of a frame transfer-type CCD solid-state image pickup device.
A image pickup section is composed of a plurality of CCD shift registers arranged in series in the vertical direction. The shift registers accumulate the information charges generated in accordance with the input light amount in each bit during the light-receiving period, and then transfers the information charges in accordance with the vertical transfer clock .phi.V during the transfer period. An accumulating section 102 is composed of a CCD shift register arranged in series with the shift registers of the image pickup section 101, and receives the information charges output from the image pickup section 101 during the transfer period to accumulate them in response to the accumulating transfer clocks .phi.S. A horizontal transfer section 103 is composed of a row (or more) of CCD shift registers arranged in series in the horizontal direction, and receives at its each bit the output from the shift register of the accumulating section 102, and outputs the information charges with a horizontal line unit in accordance with the horizontal transfer clock .phi.H. An output section 104 comprises a floating diffusion (electrically independent diffusion region) for converting the charge amount into the voltage amount and an amplifier for picking up a potential fluctuation of the floating diffusion so that the information charges output in one bit unit from the horizontal transfer section 104 into voltage value and output them as image signals. As shown in FIG. 2, for example, the output section 104 is composed of a floating diffusion 110 for receiving CCD output of the horizontal transfer section 103, a transistor 111 for resetting the potential of the floating diffusion 110 in accordance with the reset clock .phi.R, and a pair of transistors 112 and 113 which form the amplifier by being coupled to a source follower, so as to output image signals in response to the change of the charge amount of the information charges output from the horizontal transfer section 3.
FIG. 3 is a plan view showing a structure of the image pickup section 101 of the solid-state image pickup device, while FIG. 4 is a cross-sectional view cut out by the line IV--IV in FIG. 3, and FIG. 5 is a cross-sectional view cut out by the line V--V in FIG. 3.
At the light receiving region at a surface of the P-type silicon substrate 120, a plurality of channel separating regions 121 containing P-type impurities of high density therein are arranged in parallel to each other. In the same manner, also in the marginal region, a separating region 122 containing P-type impurities of high density is formed as to surround the light-receiving region. A channel region 123 between the channel separating regions 121 forms a buried channel structure by N-type impurities being diffused at the surface portion of the substrate. A plurality of transfer electrodes 125 of the first layer made of polycrystalline silicon are arranged to extend until the marginal region crossing the light-receiving region in the direction to cross the channel region 123, via an insulating layer 124 on the silicon substrate 120 having the channel separating region 121 and the channel region 123 formed thereon. On these first transfer electrodes 125, there are also provided transfer electrodes 126 of the second layer made of polycrystalline silicon to cover the gap of the first transfer electrodes 125, thereby forming two-layered structure.
During the accumulating period, a fixed potential is applied to the transfer electrodes 125 and 126 respectively such that information charges are accumulated in the light-receiving pixels formed with four transfer electrodes 125 and 126 as a unit. Further, after termination of a predetermined light-receiving period, clock pulses, for example of four phases, are applied to the transfer electrodes 125 and 126, and the information charges accumulated in each pixel are transferred to the accumulating section side along the channel region 123.
An aluminum power supply lines 128 are arranged on the transfer electrodes 125 and 126 at the marginal region via the insulating layer 127 and coupled to the transfer electrodes 125 and 126 through contact holes 129 formed in the insulating layer 127. The power supply lines 128 are provided to correspond to the number of phases of the transfer clocks supplied to the transfer electrodes 125 and 126. In the case of four-phases clock, four lines are provided, each four lines of which are regularly connected to the transfer electrodes 125 and 126.
The image pickup section of the aforementioned CCD solid-state image pickup device obtains information charges by the opto-electric effect of the incident light on the channel region 123. Therefore, there has been considered to enhance the light projecting efficiency by, for example, providing an opening in the transfer electrodes 125 and 126 and reducing the film thickness of the transfer electrodes 125 and 126. In particular, if each section is refined (micronized) to correspond to the tendency of providing higher and higher resolution, the area of the light-receiving pixel would be reduced to cause lowering of light-receiving sensitivity. To cope with this problem, the light projecting efficiency must be improved. However, if an opening is formed in the transfer electrodes 125 and 126, the shape of the electrodes would become complicated so as to make the micronizing process difficult. Therefore, this method would not be apt to the trend of higher resolution. Meanwhile, in the case of reducing the thickness of the transfer electrodes 125 and 126, piercing of the connecting portion near the marginal region of the image pickup section between power supply lines 128 and insulating layer 127 would easily arise, which would significantly reduce the reliability of the device. In addition, since the electrodes at the other regions (accumulating section and output section) which are formed by the same processes as those of the transfer electrodes 125 and 126 would become to have the same thinner thickness, there would arise lowering of the transfer efficiency of the accumulating section and horizontal transfer section, and the reduction of the characteristics of the output section.
Moreover, if the film thickness of the transfer electrodes 125 and 126 is reduced, the side marginal portion of the transfer electrode 125 of the first layer would easily float when an interlayer insulating layer for insulating the transfer electrodes of the first layer 125 from that of the second layer 126 is formed. Namely, since the interlayer insulating layer is formed by the thermal oxidation of the surface of the transfer electrode 125 formed of polycrystalline silicon, the oxide layer between the silicon substrate 120 and the transfer electrode 125 would start growing from the lateral side of the transfer electrode 125 at the time of its thermal oxidizing process, so that the side marginal portion of the transfer electrode 125 would float when the thickness of the transfer electrode 125 is thin. If such floating of the side marginal portion of the transfer electrode 125 arises, this floated portion can not be sufficiently removed by an etching process to be executed thereafter, which would cause leakage of electric current. Moreover, the effective gate length of the transfer electrode 125 would be reduced so as not to be able to provide desired characteristics, and projections would arise at the floated portion of the transfer electrodes 125 and 126, causing transfer insufficiency by the concentration of electric field at the projections.